1. Field of the Invention
This invention relates to an ultra-light sound insulator that reduces noise in a vehicle interior.
2. Brief Description of Related Arts
Japanese Patent Publication No. 2000-516175 discloses an ultra-light multifunctional insulating kit 41, which is used in vehicles to attain noise reduction and heat insulation. Sound-absorbing, sound-insulating, oscillation-damping, and heat-insulating properties are explored for use in floor insulation, end wall insulation, door covering, and inner roof covering of vehicles. The multifunctional kit 41 includes at least one areal vehicle part 11 and a multi-layer noise-reducing assembly package 42. The assembly package 42 has at least one porous spring layer 13, which is preferably formed from an open-pored foam layer. An air gap 25 is interposed between the assembly package 42 and the areal vehicle part 11. The multi-layer assembly package 42 does not have a heavy-weight layer to give the ultra-light kit 41 a suitable and optimal combination of sound-insulating, sound-absorbing, and oscillation-damping properties. The assembly package 42 also has a micro-porous stiff layer 14, which consists preferably of an open-pored fiber layer or fiber/foam composite layer. The micro-porous stiff layer 14 has a total airflow resistance from Rt=500 Nsm−3 to Rt=2500 Nsm−3, in particular from Rt=900 Nsm−3 to Rt=2000 Nsm−3, and an area-weight (weight per unit area) from mF=0.3 kg/m2 to mF=2.0 kg/m2, and in particular from mF=0.5 kg/m2 to mF=1.6 kg/m2.
According to Japanese Patent Publication No. 2000-516175, an ultra-light kit having a good acoustic efficiency can be realized when the kit is applied to a light-weight vehicle part made, for example, of aluminum or plastic. The sound insulating kit described therein is said to be lighter than a conventional sound insulation assembly by more than 50% and yet it also retains a heat-insulating property. Particularly, this type of kit can be obtained by replacing an air-impermeable heavy-weight layer used in a prior spring-mass system by a thinner micro-porous stiff fiber layer or fiber/foam composite layer. The micro-porous fiber layer has open pores but it also has a rather high resistance against airflow. For this to be accomplished, it is allegedly essential to form an air gap within the sound absorbing kit. The air gap is preferably located between the areal vehicle part and the other layers. As a result, the weight of the sound insulating system is reduced compared to a prior spring-mass system. This is preferable for the enhancement of sound absorption. A kit having a high efficiency is realized by choosing an optimum combination of sound-insulating and sound-absorbing properties. This kit can realize an extremely light structure yet it features an acoustic property that is not inferior to a conventional sound insulation assembly. Additionally, sound insulating properties are improved significantly, in a resonance domain.
However, the conventional insulating kit 41 of prior art discloses a multilayered laminated structure, a spring layer 13 having a uniform stiffness, and a stiff layer 14 having a uniform stiffness shown in FIG. 2a and FIG. 3a. The multilayers of kit 41 are made each of a different material. Accordingly, the spring layer 13, for example, in the nature of a molded foam layer, and the stiff layer 14, for example, in the nature of a highly pressed micro-porous fiber layer need to be produced separately and then combined by means such as adhesion, resulting in a greater complexity of production and a higher cost.
In addition, the tuning of sound insulating properties (transmission loss and sound absorption rate) depending on a part of vehicle where the sound insulator is to be applies needs to be done separately for each of the layers. Adjustment of sound insulating properties is thus very complicated. As described above, conventional sound insulators suffer from problems, such as complicated production processes, higher production costs, and difficult tuning of sound insulating properties.